HL-A phenotyping of approximately 2,500 samples. Maintenance of a panel of cells well defined for HL-A antigens. Assessment of serums for antibodies to HL-A and other antigens by the lymphocyte cototoxicity assay. Mixed lymphocyte cultures: Perform aaproximately 400 MLC reactions between lymphocytes from blood samples provided by the P.O. and a panel of donor accessible to the contractor, who have defined allelles at the MLC locus; and Perform approximately 100 MLC reactions between pairs of blood samples provided by the P.O.